The present invention is directed to a differential capacitive torque sensor for a continuously rotating shaft and more specifically to a sensor for automotive applications including steering.
Recent requirements from the automotive industry for reduced engine power consumption have dictated the replacement of the Power Steering Hydraulic Pump with a much more efficient electric motor geared to the steering shaft to assist the steering effort. The main problem has been sensing the effort being applied by the driver so as to know how much to assist in the steering effort.
The automotive industry attempted, with little success, to find an inexpensive method of accurately sensing the torsion in the then available 1xe2x80x3 length of the xc2xexe2x80x3 diameter steel steering shaft. The solution was to include a torsion bar within the steering shaft, thereby allowing more torsional flexibility. This movement can now be sensed using far less expensive means to determine how much effort is being applied to the steering wheel by the driver, and in turn can dictate how much power to apply to the motor for assistance.
This sensing could be accomplished with resistive elements (potentiometers), but this is a contacting technology which is prone to wear, and is electrically noisy. A non-contacting solution is therefore very much preferable. Another possibility is the use of optical encoders. While this would also perform the function, it is prohibitively expensive (especially absolute optical encoders), and the use of the light source is discouraged due to reliability considerations. In general, these and other techniques for measuring torque, currently utilize two angular displacement sensors, one on each side of the torsion rod.
A disadvantage of the differential angular displacement method is that there are two separate output sensors and the twist angle is the difference between the output signals. This is not very accurate. One solution to this problem is shown in the Lustenberger, U.S. Pat. No. 4,680,976 which utilizes an electromagnetic technique with opposed coils. This is subject to radio interference from external sources and its accuracy may not be sufficient.
It is therefore an object of the present invention to provide a differential capacitive torque sensor.
In accordance with the above object there is provided a differential capacitive torque sensor for a continuously rotating shaft where the shaft is split into first and second halves by a buried torsion bar comprising a dielectric disk having a plurality of spokes mounted for rotation with a first half of said shaft.
A pair of first and second apertured conductive disks cage the dielectric disk and are mounted for rotation with the second half of the shaft the cage shielding portions of the spokes of said dielectric disk in proportion to applied shaft torque. A pair of concentric capacitor plate rings lying in a common plane encircle the first shaft half and are juxtaposed with the first apertured conductive disk. An opposed capacitor plate encircles the second shaft half and is juxtaposed with the second apertured conductive disk. Electrical bridge means compare the capacitances formed between the pair of concentric rings and the opposed capacitor plate for determining the applied shaft torque.